The present disclosure relates generally to backlight control methodology, and more specifically, to local dimming of LED (Light Emitting Diode) backlights in LCD TVs (Liquid Crystal Display Televisions).
In a typical TFT-LCD (Thin Film Transistor-Liquid Crystal Display), an LC (Liquid Crystal) cannot illuminate by itself and requires light aids illuminating behind the LC panel from the observer's (viewer's) position. These types of light sources, known as backlights, are generally set to their maximum brightness, whereas different per-pixel grayscale values are applied to the LCs to regulate the amount of perceived brightness to observers, i.e., a pixel's grayscale works like a shutter controlling the (back-) light exposure from the pixel.
A problem with this structure is that backlight tends to leak through the panel even when pixel grayscale values are zero, ending up with poor “black level” representation. This leak (which is malignant to “black level” alone) originates from the innate structure of TFT, and it degrades the achievable Contrast Ratio (CR) in LCDs. Generally, CR is defined as the ratio of measured luminance of pure white to pure black from the panel. Accordingly, there is a need for minimization or at least reduction of backlight leak in areas with many black (or close to black) pixels, which, in turn would improve the CR for the entire picture.
To explain the concept of local dimming of LED backlights, it is helpful to understand the backlight structure of LCD TVs. Typically, a limited number of light sources, e.g., 1˜8 CCFL (Cold Cathode Florescent Lamp) backlight(s), is used in an LCD TV, even though there are, at least, more than a million pixels in any panel. This implies that only 1˜8 unit(s) of backlight is(are) independently settable to different luminance across the entire panel area. Even with Light Emitting Diode (LED) backlights (as an alternative to CCFL backlights), though the number of independently controllable units has increased, LED backlight controllable-unit granularity is much coarser than pixel granularity, mainly due to cost considerations. As a consequence, a certain area in the panel and all the pixels (which may be at different grayscale values) in that area need to be characterized to a single value such that this “composite” value determines the brightness of LED(s) underneath.
A typical LED backlight structure is shown in FIG. 1. In this FIG., 111 is the LC panel plane (shown in the foreground), and 112 is the LED backlight plane (shown in the background). In the backlight plane, each set of LEDs 113, 114, 115, 116, 117, 118, 119, 120, 121, or 122 in a rectangular grid indicates that this number of LEDs is settable as a whole in terms of brightness. The line extending between all of the LEDs in each LED group such as 113 indicates an electrical signal conductor that supplies a common amount of energy to every LED in that group. The level (e.g., the Pulse Width Modulation (PWM)) of duty ratio of the electrical signal on this conductor controls the viewer-perceived (i.e., time-averaged) brightness of all the LEDs in that group. Thus all the LEDs in any given group of LEDs have the same level of viewer-perceived brightness at any given time. But that level of brightness can be changed at various times (typically in sync with either a panel refresh rate or a time period per frame in video) by changing the PWM duty ratio of the control signal applied to those LEDs. Herein, a set of LEDs that is thus jointly controlled and settable to the same value of brightness is referred to as a “dimmable block”.
Throughout this disclosure it may sometimes be helpful to provide a graphical indication of the brightness or relative brightness of certain features. These features can be either image information, backlight illumination, or both. See especially the “Key” portion of FIG. 1 where less or more shading is used to indicate lighter or darker areas, respectively, in order from A) (lightest (like white; LEDs at maximum illumination) to J (darkest (like black; LEDs off)). In some FIGS. only different amounts of shading from this FIG. 1 key are used to indicate different amounts of brightness according to this key scheme. Sometimes this keyed shading is augmented by additional use of the capital letters A-J as in the FIG. 1 key. This keyed shading (and the associated reference letters) are generally used to indicate only relative brightness of different areas within one FIG. or a closely related group of FIGS. The same shading (and letters) may indicate different levels of brightness in different FIGS., especially FIGS. that are not closely related to one another. The depiction of only ten different possible levels (A-J) of image brightness or LED illumination is generally a simplification that is employed for convenience herein, and it will be understood that in actual practice there are typically many more levels of illumination or brightness that are employed.
One simple yet effective method to reduce the light leak through LCs for image areas that are supposed to be darker is to lower the brightness of the backlight, and this is typically done by modulating the Pulse Width Modulation (PWM) duty ratio of the illumination signal provided to the backlight underneath the darker areas. (The PWM duty ratio is, for example, the ratio between (1) the amount of time that electrical power is applied to an LED, and (2) the amount of time that electrical power is not applied to that LED in the course of pulsatile energization of the LED.) Using this approach, CR is generally improved because the viewer-perceived brightness of pure white areas is largely preserved, while the viewer-perceived brightness of pure black areas is heavily decreased. Several commercially available LCDs employ backlight control techniques by following this rule. In a popular approach, the backlight is controlled based upon sloping line 211 in FIG. 2(a). Here the backlight brightness is linearly dimmed (PWM duty ratio decreases as Gblock decreases) across the entire grayscale, where Gblock is a representative grayscale value per dimmable block. (For all of the methods that are discussed herein, including the present method, it is assumed that an image is represented with 24 bits per pixel—8 bits for each of the three color components, namely, red (R), green (G), and blue (B)—thus Gblock is also in the range 0-255 (with 0 indicating darkest or “pure” black, and with 255 indicating brightest or “pure” white). However, the methods described here are applicable to other bit depths as well, e.g., 30 bits per pixel.) In FIG. 2(a), horizontal line 212 corresponds to the absence of backlight modulation, i.e., the backlight is always fully turned on regardless of the pixel's grayscale values.
Another popular approach dims the backlight based on curve 213 in FIG. 2(b). In this case, a piece-wise linear curve 213 over three different sub-ranges/bands is used. In both cases (211 in FIGS. 2(a) and 213 in FIG. 2(b)), maximum PWM duty ratio is assigned to pure white and minimum PWM duty ratio is assigned to pure black. Hence, in a particular image that consists only of pure black and pure white, the highest CR will be achieved.